Internal combustion engines employ a fuel control system controlled by an Electronic Control Module (ECM). The basic function of the fuel control system is to control the delivery of fuel to the engine. Fuel is delivered, for example, by a Throttle Body Injection unit and on most cars, fuel injectors associated with each engine cylinder. The main control sensor for fuel control systems is the Oxygen Sensor, which is located in the engine's exhaust system. The oxygen sensor tells the ECM the amount of oxygen in the exhaust gas stream, and the ECM changes the air-fuel ratio to the engine by controlling the fuel injection. A 14.7:1 air-fuel ratio is required for efficient catalytic converter operation and fuel economy. Because of the constant measuring and adjusting of the air-fuel ratio, the system is called a “closed loop” system. FIG. 1 shows schematically a typical system of this type.
When the engine is first started, and it is above 400 rpm, the system goes into “open loop” operation. In “open loop” operation, the ECM ignores the signal from the oxygen sensor, and calculates the air-fuel ratio based upon the input from other engine sensors. When specified conditions are met the system will go into “closed loop” operation.
The system sensor, also known as a Lambda Exhaust Gas Oxygen Sensor or EGO sensor, is located in the exhaust stream, in front of the catalytic converter, usually in the exhaust manifold or the exhaust pipe and produces a signal voltage proportional to the oxygen content in the exhaust. The industry standard for the Lambda system is a zirconium dioxide sensor. A higher oxygen content across the EGO sensor tip relative to ambient oxygen, lowers the EGO' sensor's output voltage. On the other hand, lower oxygen content will raise the output voltage of the EGO sensor. Typically, the voltage range from zero to 0.1 volts (lean) to 0.9 volts or 1.0 volts (rich). The computer processor in the ECM uses the EGO sensor's voltage to adjust the air-fuel mixture, leaning it out when the EGO sensor detects a rich condition or enriching it when it detects a lean condition. The EGO sensor generates an analog voltage signal from 0 to 1. Volt, comparing the difference of the oxygen in the exhaust and the oxygen in the ambient air. The EGO sensor is based on the Lambda system concept, which is the symbol engineers use to indicate the ratio of one number to another. For air-fuel control, Lambda indicates the ratio of excess air to stoichiometric air quality. At an air-fuel ratio of 14.7:1, as much air as possible combines with the fuel. There is no excess air and there is no shortage of air, Lambda, therefore equals 1. With a lean mixture of say, 15, 16, or 17:1 there is excess air left after combustion. The Lambda air-fuel ratio of excess air to desired air is then greater then 1, It may be, say, 1.03, 1.07, 1.15 or some other number. With a rich mixture of say, 13, or 14:1, there is a shortage of air and the Lambda ratio is less than 1, such as 0.97, 0.93, 0.89, ect. With Lambda ratios less than 0.8 or greater than 1.2, a typical engine will not run. These values equate roughly to air-fuel ratios of 12.5:1 and 18:1. A typical system uses the Lambda zirconium dioxide sensor such as made by Bosch.
The zirconium dioxide EGO sensor works similar to a galvanic voltage source to generate voltages up to +1 volt. It's effective range is 0.1 to 0.9 volts (100 to 900 millivolts). When exhaust gas oxygen content is low (rich mixture), EGO sensor voltage is high (0.45 to 0.90 volts). When exhaust gas oxygen content it high (lean mixture), EGO sensor voltage is low (0.10 to 0.45 volts). FIG. 2 shows the EGO's operating range at temperatures of about 800 degrees C. (1,473 degrees F.). Notice that EGO sensor output voltage changes most rapidly near a Lambda ratio of 1, which makes it ideal for maintaining a stoichiometric ratio. The EGO Sensor must warn up to at least 300 degrees C. (572 degrees F.) before it will generate an accurate signal output.
In use, the Lambda EGO sensor develops a voltage between it's two electrodes. While all Lambda EGO sensors work on the same principle, construction may differ. Some Ego sensors have a single wire connection with ground return for it's output voltage circuit, while others may have two wire interfaces with ground return through the computer. Yet other EGO sensors may have an added built-in pre-heater, implemented through an additional single wire or wire-pair, to accelerate EGO warm-up time.
The exact location of the EGO sensor varies for different engines. Some sensors are located on the exhaust manifolds while others may be up steam of the catalytic converter.